Method of continuously casting thin strip

ABSTRACT

A method of continuously casting steel including steps of forming a casting pool of molten steel comprising a carbon content of less than 0.5% by weight on casting surfaces of a pair of internally cooled casting rolls having a nip formed between them, counter rotating the casting surfaces of the casting rolls toward each other to produce a cast steel strip moving downwardly away from the nip between the casting rolls, guiding the cast strip through a first enclosure adjacent the casting rolls as the strip moves away from the casting rolls, the first enclosure having a reducing atmosphere containing carbon monoxide and optionally hydrogen of at least 0.1%, establishing the reducing atmosphere in the first enclosure to control ingress of atmospheric air so as to maintain said atmosphere with a CO to CO 2  ratio of at least 1.5 during steady state operation.

BACKGROUND AND SUMMARY

This invention relates to the casting of metal strip by continuouscasting in a twin roll caster.

In a twin roll caster molten metal is introduced between a pair ofcounter-rotated horizontal casting rolls that are cooled so that metalshells solidify on the moving roll surfaces and are brought together ata nip between them to produce a solidified strip product delivereddownwardly from the nip between the rolls. The term “nip” is used hereinto refer to the general region at which the rolls are closest together.The molten metal may be poured from a ladle into a smaller vessel orseries of smaller vessels from which it flows through a metal deliverynozzle located above the nip, so forming a casting pool of molten metalsupported on the casting surfaces of the rolls immediately above the nipand extending along the length of the nip. This casting pool is usuallyconfined between side plates or dams held in sliding engagement with endsurfaces of the rolls so as to dam the two ends of the casting poolagainst outflow.

When casting steel strip in a twin roll caster, the strip leaves the nipat very high temperatures of the order of 1400° C. and can suffer veryrapid scaling due to oxidation at such high temperatures in an airatmosphere. Such scaling may result in a significant loss of steelproduct. Moreover, such scaling results in the need to descale the stripprior to further processing by pickling to avoid surface qualityproblems such as rolled-in scale, and causes significant extracomplexity, cost and environmental concerns.

To deal with the problem of rapid scaling of strip emerging from a twinroll strip caster, it has been proposed to enclose the newly formedstrip within a sealed enclosure, or a succession of such sealedenclosures, in which a controlled atmosphere or atmospheres ismaintained in order to inhibit oxidation of the cast strip. Thecontrolled atmosphere can be produced by charging the sealed enclosureor successive enclosures with non-oxidizing gases.

Such sealed enclosures, however, still allow ingress of some outsideair, causing oxidation that increases heat loss from the strip. Thereremains a need for further control of the atmosphere in the enclosuresdownstream of the nip.

Disclosed is a method of continuously casting steel comprising:

-   -   (a) forming a casting pool of molten steel comprising a carbon        content of less than 0.5% by weight on casting surfaces of a        pair of internally cooled casting rolls having a nip formed        between them,    -   (b) counter rotating the casting surfaces of the casting rolls        toward each other to produce a cast steel strip moving        downwardly away from the nip between the casting rolls,    -   (c) guiding the cast strip through a first enclosure adjacent        the casting rolls as the strip moves away from the casting        rolls, the first enclosure having a reducing atmosphere        containing carbon monoxide of at least 0.1% and optionally        hydrogen of at least 0.1%,    -   (d) establishing said reducing atmosphere in the first enclosure        during steady state operation to control ingress of atmospheric        air so as to maintain said atmosphere with a CO to CO₂ ratio of        at least 1.5 during.

The reducing atmosphere having and oxygen level of less than 0.5% in thefirst enclosure may also contain argon with an O₂ to Ar ratio of lessthan 18 during steady state operation. Alternatively, the reducingatmosphere in the first enclosure may have an O₂ to Ar ratio between 10and 15 during steady state operation. Alternatively, the reducingatmosphere in the first enclosure may have an oxygen level of less than0.25%.

The atmosphere in the first enclosure may have a CO to CO₂ ratio of atleast 2.5 during steady state operation.

In one alternative, the molten steel comprises a carbon content of lessthan 0.1% by weight.

Alternatively, a method of continuously casting steel may comprise:

-   -   (a) forming a casting pool of silicon killed molten steel on        casting surfaces of a pair of internally cooled casting rolls        having a nip formed between them,    -   (b) counter rotating the casting surfaces of the casting rolls        toward each other to produce a cast steel strip moving        downwardly away from the nip of the casting rolls such that iron        silicate is formed on the surface of the cast strip,    -   (c) guiding the cast strip through a first enclosure adjacent        the casting rolls as the strip moves away from the casting        rolls, the first enclosure having a reducing atmosphere        containing carbon monoxide of at least 0.1% and optionally        hydrogen of at least 0.1%, and    -   (d) establishing said reducing atmosphere in the first enclosure        during steady operation to control ingress of atmospheric air so        as to maintain said atmosphere with a CO to CO₂ ratio of at        least 1.5.

The reducing atmosphere in the first enclosure may also contain argonwith an average O₂ to Ar ratio of less than 18 during steady stateoperation. Alternatively, The atmosphere in the first enclosure may havean average O₂ to Ar ratio between 10 and 15 during steady stateoperation.

The reducing atmosphere in the first enclosure may also contain anaverage CO to CO₂ ratio of at least 2.5 during steady state operation.

The molten steel may have a carbon content of less than 0.5% by weight.Alternatively, the molten steel has a carbon content of less than 0.5%by weight.

Also disclosed is a method of continuously casting steel comprising:

-   -   (a) forming a casting pool of molten steel comprising iron and        silicon on casting surfaces of a pair of internally cooled        casting rolls having a nip formed between them,    -   (b) counter rotating the casting surfaces of the casting rolls        toward each other to produce a cast steel strip moving        downwardly away from the nip between the casting rolls such that        iron silicate is formed on the casted surface of the cast strip,    -   (c) guiding the cast strip through a first enclosure adjacent        the casting rolls as the strip moves away from the casting        rolls, the first enclosure having a reducing atmosphere        containing carbon monoxide of at least 0.1% and optionally        hydrogen of at least 0.1% to control ingress of atmospheric air        so the atmosphere in the first enclosure has a CO to CO₂ ratio        of at least 1.5 during steady state operation,    -   (d) moving the cast strip through pinch rolls and thereafter        through a second enclosure upstream of a roll mill where the        cast strip reduction is at least 10%, the atmosphere in the        second enclosure being a controlled atmosphere containing a        total of oxygen, water vapor and hydrogen of greater than 8% by        volume during steady state operation.

The method may further include the step of moving the cast strip throughan intermediate enclosure between the first enclosure and the secondenclosure, the intermediate enclosure being a reducing atmospherecontaining carbon monoxide and/or hydrogen of at least 0.1%.

A measured temperature in the first enclosure adjacent the pinch rollsmay be between about 1800 and 2400° F.

The reducing atmosphere in the first enclosure may also contain argonwith an O₂ to Ar ratio of less than 18 during steady state operation.Alternatively, the reducing atmosphere in the first enclosure may havean O₂ to Ar ratio between 10 and 15 during steady state operation.

The atmosphere in the first enclosure may also contain argon with anaverage CO to CO₂ ratio of at least 2.5 during steady state operation.

The molten steel comprises a carbon content of less than 0.5% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical side view of a twin roll caster of thepresent disclosure,

FIG. 2 is a diagrammatical plan view of the twin roll caster of FIG. 1,

FIG. 3 is a partial sectional view through a pair of casting rollsmounted in a roll cassette of the present disclosure,

FIG. 4 is a diagrammatical side view of the first enclosure of thecaster beneath the casting rolls,

FIG. 5 is a diagrammatical side view of the second and intermediateenclosures between a pinch roll and hot rolling mill,

FIG. 6 is a graph of formation of iron oxides as a function of CO in aCO+CO₂ mixture and temperature, and

FIG. 7 is a graph of formation of iron oxides on silicon killed steel asa function of CO in a CO+CO₂ mixture and temperature.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIGS. 1 through 7, a twin roll caster is illustratedthat comprises a main machine frame 10 that stands up from the factoryfloor and supports a pair of casting rolls mounted in a module in a rollcassette 11. The casting rolls 12 are mounted in the roll cassette 11for ease of operation and movement as described below. The roll cassettefacilitates rapid movement of the casting rolls ready for casting from asetup position into an operative casting position in the caster as aunit, and ready removal of the casting rolls from the casting positionwhen the casting rolls are to be replaced. There is no particularconfiguration of the roll cassette that is desired, so long as itperforms that function of facilitating movement and positioning of thecasting rolls as described herein.

As shown in FIG. 3, the casting apparatus for continuously casting thinsteel strip includes a pair of counter-rotatable casting rolls 12 havingcasting surfaces 12A laterally positioned to form a nip 18 therebetween. Molten metal is supplied from a ladle 13 through a metaldelivery system to a metal delivery nozzle 17, or core nozzle,positioned between the casting rolls 12 above the nip 18. Molten metalthus delivered forms a casting pool 19 of molten metal above the nipsupported on the casting surfaces 12A of the casting rolls 12. Thiscasting pool 19 is confined in the casting area at the ends of thecasting rolls 12 by a pair of side closures or side dam plates 20 (shownin dotted line in FIG. 3). The upper surface of the casting pool 19(generally referred to as the “meniscus” level) may rise above the lowerend of the delivery nozzle 17 so that the lower end of the deliverynozzle is immersed within the casting pool. The casting area includesthe addition of a protective atmosphere above the casting pool 19 toinhibit oxidation of the molten metal in the casting area.

The delivery nozzle 17 is made of a refractory material such as aluminagraphite. The delivery nozzle 17 may have a series flow passages adaptedto produce a suitably low velocity discharge of molten metal along therolls and to deliver the molten metal into the casting pool 19 withoutdirect impingement on the roll surfaces. The side dam plates 20 are madeof a strong refractory material and shaped to engage the ends of therolls to form end closures for the molten pool of metal. The side damplates 20 may be moveable by actuation of hydraulic cylinders or otheractuators (not shown) to bring the side dams into engagement with theends of the casting rolls.

Referring now to FIGS. 1 and 2, the ladle 13 typically is of aconventional construction supported on a rotating turret 40. For metaldelivery, the ladle 13 is positioned over a movable tundish 14 in thecasting position to fill the tundish with molten metal. The movabletundish 14 may be positioned on a tundish car 66 capable of transferringthe tundish from a heating station 69, where the tundish is heated tonear a casting temperature, to the casting position. A tundish guidepositioned beneath the tundish car 66 to enable moving the movabletundish 14 from the heating station 69 to the casting position.

The tundish car 66 may include a frame adapted to raising and loweringthe tundish 14 on the tundish car 66. The tundish car 66 may movebetween the casting position to a heating station at an elevation abovethe casting rolls 12 mounted in roll cassette 11, and at least a portionof the tundish guide may be overhead from the elevation of the castingrolls 12 mounted on roll cassette 11 for movement of the tundish betweenthe heating station and the casting position.

The movable tundish 14 may be fitted with a slide gate 25, actuable by aservo mechanism, to allow molten metal to flow from the tundish 14through the slide gate 25, and then through a refractory outlet shroud15 to a transition piece or distributor 16 in the casting position. Thedistributor 16 is made of a refractory material such as, for example,magnesium oxide (MgO). From the distributor 16, the molten metal flowsto the delivery nozzle 17 positioned between the casting rolls 12 abovethe nip 18.

The casting rolls 12 are internally water cooled so that as the castingrolls 12 are counter-rotated, shells solidify on the casting surfaces12A as the casting surfaces move into contact with and through thecasting pool 19 with each revolution of the casting rolls 12. The shellsare brought together at the nip 18 between the casting rolls to producea solidified thin cast strip product 21 delivered downwardly from thenip. FIG. 1 shows the twin roll caster producing the thin cast strip 21,which passes across a guide table 30 to a pinch roll stand 31,comprising pinch rolls 31A. Upon exiting the pinch roll stand 31, thethin cast strip may pass through a hot rolling mill 32, comprising apair of reduction rolls 32A and backing rolls 32B, where the cast stripis hot rolled to reduce the strip to a desired thickness, improve thestrip surface, and improve the strip flatness. The rolled strip thenpasses onto a run-out table 33, where it may be cooled by contact withwater supplied via water jets or other suitable means, not shown, and byconvection and radiation. In any event, the rolled strip may then passthrough a second pinch roll stand (not shown) to provide tension of thestrip, and then to a coiler.

At the start of the casting operation, a short length of imperfect stripis typically produced as casting conditions stabilize. After continuouscasting is established, the casting rolls are moved apart slightly andthen brought together again to cause this leading end of the strip tobreak away forming a clean head end of the following cast strip. Theimperfect material drops into a scrap receptacle 26, which is movable ona scrap receptacle guide. The scrap receptacle 26 is located in a scrapreceiving position beneath the caster and forms part of a sealed firstenclosure 27 as described below. The first enclosure 27 is typicallywater cooled. At this time, a water-cooled apron 28 that normally hangsdownwardly from a pivot 29 to one side in the first enclosure 27 isswung into position to guide the clean end of the cast strip 21 onto theguide table 30 that feeds it to the pinch roll stand 31. The apron 28 isthen retracted back to its hanging position to allow the cast strip 21to hang in a loop beneath the casting rolls in the first enclosure 27before it passes to the guide table 30 where it engages a succession ofguide rollers.

The sealed first enclosure 27 is formed by a number of separate wallsections that fit together at various seal connections to form acontinuous enclosure wall that permits control of the atmosphere withinthe enclosure. Additionally, the scrap receptacle 26 may be capable ofattaching with the first enclosure 27 so that the enclosure is capableof supporting a protective atmosphere immediately beneath the castingrolls 12 in the casting position. The first enclosure 27 includes anopening in the lower portion of the enclosure, lower enclosure portion44, providing an outlet for scrap to pass from the enclosure 27 into thescrap receptacle 26 in the scrap receiving position. The lower enclosureportion 44 may extend downwardly as a part of the first enclosure 27,the opening being positioned above the scrap receptacle 26 in the scrapreceiving position.

A rim portion 45 may surround the opening of the lower enclosure portion44 and may be movably positioned above the scrap receptacle, capable ofsealingly engaging and/or attaching to the scrap receptacle 26 in thescrap receiving position. The rim portion 45 is in selective engagementwith the upper edges of the scrap receptacle 26, which is illustrativelyin a rectangular form, so that the scrap receptacle may be in sealingengagement with the first enclosure 27 and movable away from orotherwise disengageable from the scrap receptacle as desired.

A lower plate 46 may be operatively positioned within or adjacent thelower enclosure portion 44 to permit further control of the atmospherewithin the enclosure when the scrap receptacle 26 is moved from thescrap receiving position and provide an opportunity to continue castingwhile the scrap receptacle is being changed for another. The lower plate46 may be operatively positioned within the first enclosure 27 adaptedto closing the opening of the lower portion of the enclosure, or lowerenclosure portion 44, when the rim portion 45 is disengaged from thescrap receptacle. Then, the lower plate 46 may be retracted when the rimportion 45 sealingly engages the scrap receptacle to enable scrapmaterial to pass downwardly through the first enclosure 27 into thescrap receptacle 26. The lower plate 46 may be in two plate portions asshown in FIGS. 1 and 4, pivotably mounted to move between a retractedposition and a closed position, or may be one plate portion as desired.A plurality of actuators (not shown) such as servo-mechanisms, hydraulicmechanisms, pneumatic mechanisms and rotating actuators may be suitablypositioned outside of the first enclosure 27 adapted to moving the lowerplate in whatever configuration between a closed position and aretracted position. When sealed, the first enclosure 27 and scrapreceptacle 26 are filled with a desired gas, such as nitrogen, to reducethe amount of oxygen in the enclosure and provide a protectiveatmosphere for the cast strip.

The first enclosure 27 may include an upper collar portion 43 supportinga protective atmosphere immediately beneath the casting rolls in thecasting position. The upper collar portion 43 may be moved between anextended position adapted to supporting the protective atmosphereimmediately beneath the casting rolls and an open position enabling anupper cover 42 to cover the upper portion of the enclosure 27. When theroll cassette 11 is in the casting position, the upper collar portion 43is moved to the extended position closing the space between a housingportion 53 adjacent the casting rolls 12, as shown in FIG. 3, and thefirst enclosure 27 by one or a plurality of actuators (not shown) suchas servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, androtating actuators. The upper collar portion 43 may be water cooled.

The upper cover 42 may be operably positioned within or adjacent theupper portion of the first enclosure 27 capable of moving between aclosed position covering the enclosure and a retracted position enablingcast strip to be cast downwardly from the nip into the first enclosure27 by one or more actuators 59, such as servo-mechanisms, hydraulicmechanisms, pneumatic mechanisms, and rotating actuators. When the uppercover 42 is in the closed position, the roll cassette 11 may be movedfrom the casting position without significant loss of the protectiveatmosphere in the enclosure. This enables a rapid exchange of castingrolls, with the roll cassette, since closing the upper cover 42 enablesthe protective atmosphere in the enclosure to be preserved so that itdoes not have to be replaced.

The casting rolls 12 mounted in roll cassette 11 are capable of beingtransferred from a set up station 47 to a casting position through atransfer station 48, as shown in FIG. 2. The casting rolls 12 may beassembled into the roll cassette 11 and then moved to the set up station47, where at the set up station the casting rolls mounted in the rollcassette may be prepared for casting. At the transfer station 48,casting rolls mounted in roll cassettes may be exchanged, and in thecasting position the casting rolls mounted in the roll cassette areoperational in the caster. A casting roll guide is adapted to enable thetransfer of the casting rolls mounted in the roll cassette between theset up station and the transfer station, and between the transferstation and the casting position. The casting roll guides may compriserails on which the casting rolls 12 mounted in the roll cassette 11 arecapable of being moved between the set up station and the castingposition through the transfer station. Rails 55 may extend between theset up station 47 to the transfer station 48, which may include aturntable 58, and rails 56 may extend between the transfer station 48 tothe casting position. The casting rolls mounted in a roll cassette maybe raised or lowered into the casting position. In one embodiment, theroll cassette 11 may include wheels 54 capable of supporting and movingthe roll cassette on the rails 55, 56.

The roll cassette 11 comprises a cassette frame 52, roll chocks 49capable of supporting the casting rolls 12 and moving the casting rollson the cassette frame, and the housing portion 53 positioned beneath thecasting rolls capable of supporting a protective atmosphere in the firstenclosure 27 immediately beneath the casting rolls during casting. Thehousing portion 53 is positioned corresponding to and sealingly engagingan upper portion of the first enclosure 27 for enclosing the cast stripbelow the nip.

A roll chock positioning system is provided on the main machine frame 10having two pairs of positioning assemblies 50 that can be rapidlyconnected to the roll cassette adapted to enable movement of the castingrolls on the cassette frame 52, and provide forces resisting separationof the casting rolls during casting. The positioning assemblies 50 moveat least one of the casting rolls 12 toward or away from the othercasting roll to provide a desired nip, or gap between the rolls in thecasting position.

The casting rolls 12 are counter-rotated through drive shafts by anelectric motor and transmission (not shown) mounted on the main machineframe. The casting rolls 12 have copper peripheral walls formed with aninternal series of longitudinally extending and circumferentially spacedwater cooling passages, supplied with cooling water through the rollends from water supply ducts in the shaft portions, which are connectedto water supply hoses through rotary joints (not shown). The castingrolls 12 may be between about 450 and 650 millimeters. Alternatively,the casting rolls 12 may be up to 1200 millimeters or more in diameter.The length of the casting rolls 12 may be up to about 2000 millimeters,or longer, in order to enable production of strip product of about 2000millimeters width, or wider, as desired in order to produce stripproduct approximately the width of the rolls. Additionally, at least aportion of the casting surfaces may be textured with a distribution ofdiscrete projections, for example, random discrete projections asdescribed and claimed in U.S. Pat. No. 7,073,565 and having the tapereddistribution of surface roughness described herein. The casting surfacemay be coated with chrome, nickel, or other coating material to protectthe texture.

As shown in FIGS. 3 and 5, cleaning brushes 36 are disposed adjacent thepair of casting rolls, such that the periphery of the cleaning brushes36 may be brought into contact with the casting surfaces 12A of thecasting rolls 12 to clean oxides from the casting surfaces duringcasting. The cleaning brushes 36 are positioned at opposite sides of thecasting area adjacent the casting rolls, between the nip 18 and thecasting area where the casting rolls enter the protective atmosphere incontact with the molten metal casting pool 19. Optionally, a separatesweeper brush 37 may be provided for further cleaning the castingsurfaces 12A of the casting rolls 12, for example at the beginning andend of a casting campaign as desired.

A knife seal 65 may be provided adjacent each casting roll 12 andadjoining the housing portion 53. The knife seals 65 may be positionedas desired near the casting roll and form a partial closure between thehousing portion 53 and the rotating casting rolls 12. The knife seals 65enable control of the atmosphere around the brushes, and reduce thepassage of hot gases from the enclosure 27 around the casting rolls. Theposition of each knife seal 65 may be adjustable during casting bycausing actuators such as hydraulic or pneumatic cylinders to move theknife seal toward or away from the casting rolls.

The casting rolls 12 are internally water cooled so that as the castingrolls 12 are counter-rotated, shells solidify on the casting surfaces12A as the casting surfaces rotate into contact with and through thecasting pool 19. During casting, metal shells formed on the castingsurfaces of the casting rolls are brought together at the nip to delivercast strip downwardly from the nip into the first enclosure 27. Betweenthe casting rolls and pinch roll stand 31, the newly formed steel stripis enclosed within the first enclosure 27 defining a sealed space oratmosphere. The first enclosure 27 is formed by a number of separatewall sections which fit together at various seal connections to form acontinuous enclosure wall.

The first enclosure 27 further comprises a wall section 41 disposedabout the guide table 30 and connected to the frame of the pinch rollstand 31. Accordingly, the strip exits the first enclosure 27 by passingbetween the pair of pinch rolls 31A and passes into a intermediateenclosure denoted generally as 61 supporting an atmosphere 68.

After passing through pinch roll stand 31, the strip 21 is supported bythe guide table 30 to the rolling mill 32 as shown in FIG. 5. Ananti-crimping guide roll 70 may be located immediately in advance of therolling mill 32, operable to be raised and lowered to lift the caststrip out of its straight line horizontal path so as to pass around theanti-crimping roll and to be wrapped about the upper reduction roll 32Ain advance of the nip between the reduction rolls 32A.

To hold the strip down on the guide table 30 when the anti-crimping roll70 is raised, an upper pass line roll 72 is brought downwardly to engagethe strip against the guide table 30.

The intermediate enclosure 61 extends generally to the pass line roll72. The atmosphere 68 of the intermediate enclosure 61 may be separatefrom the first enclosure 27, where the strip can be held in a separateatmosphere 68 in the intermediate enclosure 61. Alternatively, theatmosphere 68 in the intermediate enclosure 61 may be substantially thesame as the atmosphere in the first enclosure 27.

The cast strip 21 is enclosed in a second enclosure 74 between the passline roll 72 and the hot rolling mill 32 supporting a second enclosureatmosphere 76. The second enclosure 74 may be fitted with water spraynozzles operable to spray a fine mist of water droplets adjacent thesurface of the steel strip as it passes through the second enclosure,and thereby to generate steam within the second enclosure while tendingto avoid liquid water contact with the steel strip. The nozzles may beoperable with a gas propellant to produce a fine mist of water. In onealternative, the gas propellant may be an inert gas such as nitrogen.The water may be supplied at around 100-500 kPa pressure, although thepressure of the water is not critical. Accordingly, the nozzles may beset up to produce a fine mist spray across the width of the strip togenerate steam within the second enclosure 74.

In operation of the illustrated caster, the first enclosure 27,intermediate enclosure 61, and second enclosure 74 may initially bepurged with nitrogen gas prior to commencement of casting. Prior tocasting, the water sprays are activated so that as soon as the hot strippasses into the second enclosure 74 steam is generated within thatenclosure so as to produce a positive pressure preventing ingress ofatmospheric air. The supply of nitrogen may be terminated in the secondenclosure 74 after commencement of casting.

As used in the specification and claims herein, “seal”, “sealed”,“sealing”, and “sealingly” in reference to the scrap receptacle 26,first enclosure 27, intermediate enclosure 61, second enclosure 74 andrelated features is not a complete seal so as to prevent leakage, butrather is usually less than a perfect seal as appropriate to allowcontrol and support of the atmosphere within the enclosure as desiredwith some tolerable leakage. As such, the supply of nitrogen into thefirst and intermediate enclosures may be controlled to limit the amountof air ingress.

The cast strip will take up oxygen present in the first, intermediateand second enclosures forming scale on the strip. The scale on the stripsurface increases the emissivity of the surface of the strip, i.e. therelative ability of the surface to emit energy by radiation. Radiantheat transfer increases with increasing emissivity, and thereby thestrip temperature reduces as scale forms on the strip surface. Tocontrol the oxidation of the strip surface and maintain a desiredtemperature at the hot rolling mill, a reducing atmosphere may bemaintained in the first enclosure 27 and the intermediate enclosure 61.Controlling oxidation of the surface of the strip controls heat transferby radiation, and thereby the temperature drop of the strip before thehot rolling mill. Additionally, limiting scale formation reduces surfaceimperfections caused by rolled-in scale.

The measured temperature in the first enclosure 27 adjacent the pinchrolls 31 may be between about 1800 and 2400° F. As shown by the lineidentified by reference “A” in FIG. 6, at a temperature of about 2200°F., in an atmosphere containing CO and CO₂ oxidation will besubstantially reduced where the CO/CO₂ ratio is greater than about 3,i.e. 75% CO in the CO+CO₂ mixture for Fe°. However, for steelsdeoxidized using silicon, i.e. silicon killed steels, the iron reactswith silicon oxides to form fayalite, FeSiO₂, and/or other ironsilicates forming a protective layer on the steel surface. For siliconkilled steels, in a CO+CO₂ atmosphere higher levels of CO₂ may betolerated while maintaining a reducing atmosphere. As shown in FIG. 7,the protective layer of iron silicates inhibits the oxidation of thesteel. The point at which FeO forms along the C+CO₂=2CO equilibrium lineis identified as point “B” in FIG. 7. This point is approximately 60% COin the CO+CO₂ mixture. As such, there is substantially reduced oxidationat a CO/CO₂ ratio of greater than about 1.5, i.e. at least 60% CO in theCO+CO₂ mixture for silicon killed steel.

The atmosphere in the first enclosure 27 may include CO₂ with theingress of air into the first enclosure. CO is present in the firstenclosure 27 as shown below in TABLE 1 from reaction of oxygen andcarbon from the molten steel above the nip exiting the steel adjacentthe nip. Some amount of CO may be formed from CO₂ reacting with carbonon the surface of the steel. The atmosphere in the first enclosure 27may have a CO to CO₂ ratio of at least 1.5 during steady stateoperation. Alternatively, the atmosphere in the first enclosure has a COto CO₂ ratio of at least 2.5 during steady state operation. In yetanother alternative, the atmosphere in the first enclosure has a CO toCO₂ ratio of at least 3 during steady state operation. As used in thespecification and claims herein, “steady state operation” does not meanstrictly unchanging over time, but is instead an average of datacollected during a period of normal casting operation in which nosignificant caster or casting parameters are changed, for exampleremoval of the scrap box or change of casting speed.

Optionally, hydrogen may be provided in the first enclosure to provide areducing atmosphere. The amount of hydrogen may be greater than 0.1% inthe first enclosure 27.

Experimental data showing the average amounts of carbon monoxide andcarbon dioxide in the enclosures are shown in TABLE 1 during casting ofsilicon killed steel. As shown in TABLE 1, the CO/CO₂ ratio is betweenabout 2 and 6 in the first enclosure 27 and the intermediate enclosure61 providing a reducing atmosphere.

TABLE 1 Carbon Monoxide Carbon Dioxide Oxygen (O₂) Water (H₂O) Hydrogen(H₂) (CO) (CO₂) (vol %) (vol %) (vol %) (vol %) (vol %) Casting Pool<0.05% <0.05% 0.05 to 0.5%  0.1 to 0.5%  <0.1% First Enclosure, 0.1 to0.25% <0.05% 0.05 to 0.25% 0.1 to 0.3% <0.05% Loc. #1 First Enclosure,0.1 to 0.25% <0.05% 0.05 to 0.25% 0.1 to 0.3% <0.05% Loc. #2Intermediate 0.1 to 0.5%   <0.5% 0.05 to 0.25% 0.1 to 0.3% <0.05%Enclosure Second 2 to 8%  5 to 10% 0.05 to 0.25% <0.05% <0.05% Enclosure

The oxygen content in the first enclosure 27 is less than 0.5% byvolume. Alternatively, the oxygen is less than 0.25% by volume in thefirst enclosure 27. Air ingress into the first enclosure 27 adds oxygenand argon to the reducing atmosphere. Nitrogen may be introduced intothe first enclosure 27 such that the O₂ to Ar ratio is less than 18during steady state operation while maintaining desired oxygen levelsbelow 0.5%. Alternatively, the reducing atmosphere in the firstenclosure having an oxygen level of less than 0.5% has an O₂ to Ar ratiobetween about 10 and 15 during steady state operation. When excess airenters the enclosure, additional oxygen and argon enter the enclosure.However, excess oxygen in the air is consumed by the steel, lowering theO₂ to Ar ratio. Moisture in the first enclosure may be less than 0.2%water vapor by volume to further inhibit oxidation.

As discussed above, water is provided in the second enclosure 74. Adesired amount of oxidation may be provided on the strip to improve thelife of the reduction rolls 32A and to improve the cast strip surface.The second enclosure atmosphere 76 may include hydrogen, water, andoxygen. The total of oxygen, water vapor and hydrogen in the secondenclosure atmosphere 76 may be greater than 8% by volume.

In one application, the cast strip may include the followingcomposition, with the balance being iron and inevitable impurities (byweight):

Carbon 0.02-0.04% Manganese 0.6-0.9% Silicon 0.15-0.24% Sulfur0.001-0.003% Phosphorus  0.01-0.018% Copper 0.26-0.37% Chromium0.09-0.17% Nickel 0.09-0.16% Molybdenum 0.03-0.04%

Alternatively, the composition of the cast strip includes a carboncontent of less than 0.5% by weight. In yet another alternative, thecomposition may have a carbon content of less than 0.2% by weight.

While the invention has been described with reference to certainembodiments it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiments falling within the scope of the appended claims.

1. A method of continuously casting steel comprising: (a) forming acasting pool of molten steel comprising a carbon content of less than0.5% by weight on casting surfaces of a pair of internally cooledcasting rolls having a nip formed between them, (b) counter rotating thecasting surfaces of the casting rolls toward each other to produce acast steel strip moving downwardly away from the nip between the castingrolls, (c) guiding the cast strip through a first enclosure adjacent thecasting rolls as the strip moves away from the casting rolls, the firstenclosure having a reducing atmosphere containing carbon monoxide of atleast 0.1% and optionally hydrogen of at least 0.1%, (d) establishingsaid reducing atmosphere in the first enclosure during steady stateoperation to control ingress of atmospheric air so as to maintain saidatmosphere in the first enclosure with a CO to CO₂ ratio of at least1.5.
 2. The method of continuously casting steel as claimed in claim 1where the reducing atmosphere in the first enclosure also contains argonwith an O₂ to Ar ratio of less than 18 during steady state operation. 3.The method of continuously casting steel as claimed in claim 2 where thereducing atmosphere in the first enclosure has an O₂ to Ar ratio between10 and 15 during steady state operation.
 4. The method of continuouslycasting steel as claimed in claim 1 where the atmosphere in the firstenclosure has a CO to CO₂ ratio of at least 2.5 during steady stateoperation.
 5. The method of continuously casting steel as claimed inclaim 1 where the molten steel comprises a carbon content of less than0.1% by weight.
 6. A method of continuously casting steel comprising:(a) forming a casting pool of silicon killed molten steel on castingsurfaces of a pair of internally cooled casting rolls having a nipformed between them, (b) counter rotating the casting surfaces of thecasting rolls toward each other to produce a cast steel strip movingdownwardly away from the nip of the casting rolls such that ironsilicate is formed on the surface of the cast strip, (c) guiding thecast strip through a first enclosure adjacent the casting rolls as thestrip moves away from the casting rolls, the first enclosure having areducing atmosphere containing carbon monoxide of at least 0.1% andoptionally hydrogen of at least 0.1%, and (d) establishing said reducingatmosphere in the first enclosure during steady state operation tocontrol ingress of atmospheric air so as to maintain said atmospherewith a CO to CO₂ ratio of at least 1.5.
 7. The method of continuouslycasting steel as claimed in claim 6 where the reducing atmosphere in thefirst enclosure also contains argon with an average O₂ to Ar ratio ofless than 18 during steady state operation.
 8. The method ofcontinuously casting steel as claimed in claim 7 where the atmosphere inthe first enclosure has an average O₂ to Ar ratio between 10 and 15during steady state operation.
 9. The method of continuously castingsteel as claimed in claim 6 where the reducing atmosphere in the firstenclosure also contains an average CO to CO₂ ratio of at least 2.5during steady state operation.
 10. The method of continuously castingsteel as claimed in claim 6 where the molten steel comprises a carboncontent of less than 0.1% by weight.
 11. A method of continuouslycasting steel comprising: (a) forming a casting pool of molten steelcomprising iron and silicon on casting surfaces of a pair of internallycooled casting rolls having a nip formed between them, (b) counterrotating the casting surfaces of the casting rolls toward each other toproduce a cast steel strip moving downwardly away from the nip betweenthe casting rolls such that iron silicate is formed on the castedsurface of the cast strip, (c) guiding the cast strip through a firstenclosure adjacent the casting rolls as the strip moves away from thecasting rolls, the first enclosure having a reducing atmospherecontaining carbon monoxide of at least 0.1% and optionally hydrogen ofat least 0.1% to control ingress of atmospheric air so the atmosphere inthe first enclosure has a CO to CO₂ ratio of at least 1.5 during steadystate operation, (d) moving the cast strip through pinch rolls andthereafter through a second enclosure upstream of a roll mill where thecast strip reduction is at least 10%, the atmosphere in the secondenclosure being a controlled atmosphere containing a total of oxygen,water vapor and hydrogen of greater than 8% by volume during steadystate operation.
 12. The method of continuously casting steel as claimedin claim 11 comprising in addition moving the cast strip through anintermediate enclosure between the first enclosure and the secondenclosure, the intermediate enclosure being a reducing atmospherecontaining carbon monoxide and/or hydrogen of at least 0.1%.
 13. Themethod of continuously casting steel as claimed in claim 11 with ameasured temperature in the first enclosure adjacent the pinch rollsbetween 1800 and 2400° F.
 14. The method of continuously casting steelas claimed in claim 11 where the reducing atmosphere in the firstenclosure also contains argon with an O₂ to Ar ratio of less than 18during steady state operation.
 15. The method of continuously castingsteel as claimed in claim 11 where the reducing atmosphere in the firstenclosure has an O₂ to Ar ratio between 10 and 15 during steady stateoperation.
 16. The method of continuously casting steel as claimed inclaim 11 where the atmosphere in the first enclosure also contains argonwith an average CO to CO₂ ratio of at least 2.5 during steady stateoperation.
 17. The method of continuously casting steel as claimed inclaim 11 where the molten steel comprises a carbon content of less than0.1% by weight.